The invention relates generally to high-voltage, solid-state switching devices, and more particularly to such switching devices which provide low on-state losses, support high current densities, and have low volume/mass characteristics.
Presently available solid-state switching devices represent a compromise between low losses and desirable volume/mass characteristics. It is possible to design a relatively low-loss switch with existing solid-state devices; however, such a switch will generally be quite large and heavy. On the other hand, a light, compact solid-state switch will generally offer relatively higher losses. Unfortunately, the existing devices are not able to support low volume/mass characteristics and simultaneously provide low on-state losses.
Furthermore, existing devices must be fabricated on the basis of other design tradeoffs. For example, solid-state devices are available which offer either: (1) current interruption capability; or (2) efficient, high-power operation. However, these two design goals have generally remained mutually exclusive. If high-voltage (i.e., greater than 1000 volts), high current density, low forward voltage drop, and efficient operation are desired, a reverse blocking triode thyristor (commonly known as a silicon-controlled rectifier or SCR) can be used. However, where current interruption capability is desired, the use of a less efficient more costly device, such as a transistor, is mandated if solid-state components are to be used. Unfortunately, currently available transistors are not able to operate as efficiently as SCR's at comparable power levels.
One approach to designing high-voltage, high-current switching devices involves the use of multiple devices wired in parallel-series configurations. An array of field-effect transistors (FET's) or insulated gate bipolar transistors (IGBT's) may be employed, resulting in intrinsic current sharing among the devices. However, the voltage drop across the multiple series devices is prohibitive for many practical applications.
A second approach to designing a high voltage switching device employs two silicon-controlled rectifiers (SCR's) and a commutation capacitor. An SCR is a thyristor that can be triggered into conduction in only one direction. Employing two SCR's as switching elements takes advantage of the lower forward voltage drop of the SCR relative to transistor devices. However, once current starts flowing through the SCR, the SCR cannot be turned off until the current flow approaches zero. Thus, a two-state switching configuration is employed to provide for current interruption capabilities. A capacitor is wired in series with a first SCR, and the resulting capacitor-SCR combination is wired in parallel with a second SCR. A voltage source is connected to the capacitor to provide negative bias when it is desired to interrupt the flow of current through the second SCR. This is accomplished by transferring current from the second SCR to the first SCR, a process referred to as "commutation". Unfortunately, the commutation process requires a relatively large capacitor and a relatively hefty power supply. Furthermore, a negative voltage is impressed across the output terminals of the switch during commutation, when the current is being transferred from one SCR to the other. Since the switch may be wired to a power supply, the output terminals of the switch may be used to supply power to a load device. Hence, it may be said that the output terminals feed power rails which supply power to the load. An additional drawback of this approach is that, as a practical matter, it is very difficult to obtain adequate current sharing between the system components. The result being an increased vulnerability to early device failure.
A third approach to designing high voltage switching devices makes use of a relatively new device known as an MCT, or a metal-oxide silicon-controlled thyristor. A group of MCT's may be wired in parallel-series combinations to provide for the sharing of current. However, a relatively large commutation capacitor is required. It is difficult to obtain a reasonable level of current distribution among the various individual MCT devices which together comprise an MCT array. Furthermore, a series combination of MCT's must be used to achieve a high voltage switch. Finally, the MCT is a relatively new device--it is not on the market at the present time insofar as applicant is aware. Although the device has a promising potential for switching applications, at present, more development is required in this area to produce an MCT with a low forward voltage drop.
A fourth approach to designing high-voltage switching devices uses a gate-turn-off thyristor or GTO. GTO devices offer output current switching that can be controlled or turned off by a large current counter-pulse (approximately 30% of the device current) into the device gate. The GTO is wired in parallel with a commutation capacitor which must sustain the full load current during switching. Like the SCR the GTO can be manufactured at high-current and high-voltage ratings but it is less efficient and it requires a large commutation capacitor and a high current gating power supply to turn off.
From the foregoing it is evident that what is needed is a high-voltage, solid-state switch which offers low on-state resistance and low volume/mass characteristics, that is capable of supporting high current densities. The switch should offer current interruption capabilities. Large, bulky commutation capacitors should not be required because the switch may be employed in applications such as aircraft weapons systems, where size and weight are at a premium. If the switch employs a plurality of solid-state devices arranged in an array, current should be distributed fairly evenly among the various devices. Additionally, some applications may require reverse-blocking capability or polarity protection. Ideally, the switch should not impress a negative voltage upon the power rails during commutation because the presence of a negative voltage renders such switching systems unsuited to many practical applications where polarity-sensitive components might be damaged or destroyed.